1. Field of the Invention
The present invention relates to an image decoder for discrete cosine transform and motion compensation, and more particularly to an image decoder which separately performs a process for inverse quantization and inverse discrete cosine transform and a process for motion compensation with respective blocks and then compensates for the image data scanning difference generated during the respective processes, thereby enabling a stable motion compensation process in the contour portion of subpictures.
2. Description of the Prior Art
Generally, in an HDTV or MPEG standard image signal processing system, an image encoder or an image decoder for discrete cosine transform and motion compensation applied to image compression/expansion system requires a high clock frequency (above 50 MHz sampling clocks in case of HDTV) for real time processing of a video signal. Thus, a parallel processing method with division of the picture is applied in order to overcome the problem of hardware construction due to the real time processing of video signals with such a high clock frequency. That is, after a single picture to be signal-processed is divided into n subpictures, n image decoders are provided in parallel to individually perform a process for inverse quantization and inverse discrete cosine transform and a process for motion compensation on the respective subpictures. The expanded video signal can be obtained by adding the outputs of the respective decoders. As described above, the image decoders on n subpictures divide respective video signals, thereby reducing sampling clock frequency to 1/n and simply performing real time processing of video signal.
FIG. 1 is a schematic block diagram of a conventional image decoder, where the decoder processes a subpicture corresponding to 1/n of the overall picture. The decoder comprises a variable length decoder 1 for decoding the compressed video signal transmitted from image the encoder as a variable length code, an inverse quantization and inverse discrete cosine transform section 2 for restoring the video signal provided from variable length decoder 1, a frame memory 3 for storing the video signal in order to motion compensate for the signal, a motion compensating section 4 for performing motion compensation by reading out the video signal stored in frame memory 3, and an adder 5 for adding the inverse quantized and inverse discrete cosine transformed video signal to the motion compensated video signal. The image decoder takes one subpicture among n subpictures P1 to Pn divided from one frame shown in FIG. 2. That is, an image decoder on the overall picture is completed by providing n image decoders in FIG. 1 in parallel on n subpictures.
Operation of the above image decoder on one subpicture among n subpictures will be explained as follows.
Variable length decoder 1 decodes an input video signal and then provides the decoded signal to inverse quantization and inverse discrete transform section 2 which inverse quantizes and inverse discrete cosine transforms the decoded video signal and provides the inversely processed signal to adder 5. The video signal from adder 5 is stored in frame memory 3 and is read out by motion compensating section 4 and then is returned to adder 5 as a motion compensated video signal. Then, adder 5 adds the inverse quantized and inverse discrete cosine transformed video signal to the motion compensated video signal, thereby providing the final expanded video signal.
However, in the image decoder according to the division of picture, inverse quantization and inverse discrete cosine transform are processed easily but there may be a problem in processing motion compensation. That is, if information on motion vector MV crosses the contour portions of subpicture Pn as shown in FIG. 3, each decoder cannot be independently parallel-processed on respective subpictures. Accordingly, separate control logic circuits and memories for motion compensation in contour portions of subpictures are required. Thus the effect of a parallel processing method for real time processing with a low speed clock is reduced. Also, information on motion compensation which crosses the contour portions of subpictures causes image processing with a low frequency to be unstable so that the picture quality deteriorates.